RFID is an auto-recognition system for reading/writing (specifically, retrieving, registering, deleting, or updating) individual information from/to a person or a substance memorized in a medium called an IC tag by using wireless communications. FIG. 18 is a schematic view showing an example of the system.
Two devices are necessary in order to provide the RFID. One is a reader/writer 100 and another is an IC tag 120. The reader/writer 100 writes information to a memory 121a incorporated in an IC chip 121 of the IC tag 120, or reads the information written in the memory 121a. The IC tag 120 obtains an electric power for its operation by rectifying part of carrier waves when it is a passive type IC tag having no internal battery. Thus, the IC tag 120 can communicate with the reader/writer 100, write data to the memory 121a, and modulate data for transmission.
An antenna 122 is incorporated in the IC tag 120 in order to maintain the communications between the reader/writer 100 and the IC tag 120 at a long distance. A dipole antenna and a patch antenna are typically used for the IC tag 120 utilizing a microwave frequency range (e.g., the 960 MHz range and the 2.45 GHz range).
FIG. 19 is a schematic view showing a dipole antenna 219. The dipole antenna 219 can be easily manufactured by printing, because it has a simple structure having two linear metals 130 and 131. Therefore the dipole antenna 219 is widely used as an inexpensive antenna for the IC tag.
FIGS. 20A, 20B are schematic views showing a patch antenna 220. The patch antenna 220 has a structure in which two metal plates (i.e., a ground metal plate 140 and a patch metal plate 141) are arranged parallel to each other together with a dielectric body 142 therebetween.
Directivity is used as an indicator of characteristics of an antenna. The directivity indicates which direction the antenna can cover as a communications area (or a recognition area) and how much power the antenna can provide. FIGS. 21 and 22 are schematic views showing the directivity of the dipole antenna 219 and the patch antenna 220, respectively. As shown in the dashed lines in FIGS. 21 and 22, the dipole antenna 219 is capable of transmitting electrical waves to its upper side and lower side in FIG. 21, while the patch antenna 220 is capable of transmitting electrical waves mainly to its upper side (toward the patch metal plate 141) in FIG. 22. From the viewpoint of a recognition area of the reader/writer 100, the dipole antenna 219 can be recognized in a broader angle than the patch antenna 220. In other words, if IC tags with the dipole antenna 219 and IC tags with the patch antenna 220 are disposed in random orientations, the IC tags with the dipole antenna 219 can be recognized more easily.
In many cases, IC tags are used by being attached to an object. This object is, for example, a dielectric material such as a PET bottle, or a metal object such as a metal container. The object, especially the metal object, tends to degrade the characteristics of the antennas. Hereafter, the functions of the dipole antenna 219 and the patch antenna 220 will be described.
As shown in FIG. 23, when the dipole antenna 219 is located near a metal 132, an incident wave 135 advancing to the metal 132 and an reflection wave 136 reflected by the metal 132 are generated. At the edge of the metal 132, the incident wave 135 is in the opposite phase to the reflection wave 136, because of a boundary condition that the electric field on the metal 132 is zero. Therefore, when the dipole antenna 219 and the metal 132 are located very close to each other, a transmission wave 137 advancing upward in FIG. 23 from the dipole antenna 219 is also in almost the opposite phase to the reflection wave 136. Therefore, the transmission wave 137 and the reflection wave 136 are canceled by each other and thereby an electrical wave is not transmitted upward from the dipole antenna 219. In other words, the dipole antenna 219 does not function when it is located near the metal 132.
In JP-2004-164055 A, in order to solve the problem, a spacer is inserted between an antenna and a metal to keep an interval between them within a range λ/4±λ/8, wherein the value λ is wavelength of waves from the antenna. By keeping the interval at the λ/4, a reflection wave reflected by the metal is in the same phase as that of the wave from the antenna, at the location of the antenna. However, as for the UHF (960 MHz) frequency range, the antenna with the spacer is not practical because λ/4 roughly equals no less than 8 cm.
In contrast, since the patch antenna 220 has a ground (i.e., the ground metal plate 140) as shown in FIG. 24, the characteristics of transmission of the patch antenna 220 is hardly changed even if the ground metal plate 140 and a metal 143 come in contact with each other.
As described above, the patch antenna 220 is superior to the dipole antenna 219 in the case that they are located near the metal.
Hereafter, matching between the IC chip and the antenna will be described. The above description on the transmission from the antenna is made under an assumption that impedance matching between the IC chip and the antenna is successfully made. In most of ordinary wireless communicators, impedance between a transmitting/receiving device and an I/O port of an antenna is predetermined to be, for example, 50Ω. In contrast, the IC chip for the IC tag does not have a matching circuit, because the IC chip is required to be manufactured with a low cost. Therefore, it is necessary to adjust the impedance of the antenna for the IC tag for achieving the matching with the IC chip. Specifically, when the impedance of the IC chip is (R−jX) [Ω], the impedance of the antenna has to be (R+jX) [Ω] which is the complex conjugate of the impedance of the IC chip.